(1) Field of the Invention
The present invention relates to a metal halide lamp and to a lighting apparatus adopting the metal halide lamp. In particular, the present invention relates to a technology for obtaining a desired color characteristic as well as preventing thin tubes from cracking, in a ceramic metal halide lamp capable of being dimmed.
(2) Related Art
Recently, from a point of view of power saving, metal halide lamps that can be dimmed are desired. Specifically, such lamps are to be normally lit at high lamp power (rated power), while having a function of being lit at lower lamp power when not so much light is required.
In a metal halide lamp whose envelope is made of translucent ceramic (hereinafter referred to as “ceramic metal halide lamp”), various types of rare earth metal halides are enclosed, such as dysprosium halide, thulium halide, holmium halide, cerium halide, and praseodymium halide. The envelope is made up of: a main tube in which a pair of electrodes are positioned; and two thin tubes provided at both ends of the main tube. In each of the thin tubes, a power feeder whose tip is provided with a corresponding one of the electrodes is inserted.
When such a ceramic metal halide lamp is lit using a dimming-control function, the color temperature (K) differs between a condition under which the lamp is lit under low lamp power, and a condition under which the lamp is lit under high lamp power. This happens in the following way. When a ceramic metal halide lamp is lit under low lamp power, a temperature of the coolest-spot in the arc tube becomes lower than when the lamp is lit under high lamp power, and accordingly the vapor pressure of each luminous material becomes lower than under high lamp power. However, the ratio of vapor-pressure reduction is different for each luminous metal, and so the distribution of light emission spectrum will change. For example, for a conventional ceramic metal halide lamp of lamp power of 150 W in which dysprosium iodide (DyI3), thulium iodide (TmI3), holmium iodide (HoI3), and thallium iodide (TlI) are enclosed as luminous materials, the color temperature was 4300K, and Duv (displacement of chromaticity coordinates (u,v) from the Planckian locus) was 0. When this conventional lamp was lit under dimming control of 60% (i.e. lamp power of 90 W), the color temperature was 5100K, and the Duv was 20. This is attributed to a fact that the reduction ratio of the vapor pressure is smaller for thallium iodide than for dysprosium iodide, thulium iodide, and holmium iodide.
In view of this, a ceramic metal halide lamp that uses magnesium halide instead of thallium iodide has been proposed, so as to realize even reduction of vapor pressure for each luminous material when the lamp is lit under low lamp power (e.g. patent reference 1: Japanese Laid-open patent application No. 2002-42728).
The inventors of the present invention, in view of this patent reference 1, produced and evaluated ceramic metal halide lamps that contain magnesium iodide (MgI2) as aluminous material instead of thallium iodide, in addition to dysprosium iodide, thulium iodide, and holmium iodide. The ceramic metal halide lamps have maximum lamp power of 150 W and minimum lamp power of 90 W.
Note that the content of the magnesium iodide is controlled to be 5%-50% of the total molar quantity of the metal halides.
A life test was conducted to thus produced lamps, by lighting them with a maximum lamp power of 150 W, without performing dimming control. The result is unexpected in a sense that the lamps stopped lighting up after about 4500 hours has passed after the lighting start, where the lamp rated life is 9000 hours. During examination for the cause, cracks were found at one of the ends of the thin tubes that is nearer the main tube. Therefore, it is considered that the leakage due to such cracks has caused the non-lighting of the lamps. Furthermore, the cracks are considered to have occurred at one of the ends of the thin tubes which is nearer the main tube, in the following manner. In an inner surface of one of the ends of the thin tubes that is nearer the main tube, the ceramic constituting the thin tubes reacted with the luminous materials (i.e. luminous metal) to be eroded. As a result, the ceramic lost its mechanical strength. Therefore, the selection and composition of luminous materials disclosed in the patent reference 1 are considered to have facilitated the reaction between the luminous materials and the ceramic, thereby causing the thin tubes to crack.
However, there is no practical substitute for the selection of luminous material as disclosed in the patent reference 1, to been closed in a ceramic metal halide lamp capable of being dimmed. Reduction in content of the luminous materials has been also considered so as to restrain the reaction with the ceramic. However, this is not a practical measure either, because it is expected to reduce the vapor pressure of each luminous material during the lamp slighting, which would impair the desired color characteristics.